The applicants proposed to investigate breast imaging based on the novel ultrasound-modulated optical tomography technique to provide a non-invasive, nonionizing, inexpensive diagnostic tool for early breast cancer detection. A focused ultrasonic wave defines the imaging resolution and overcomes the poor localization of light in biological tissues caused by the strong scattering of light in tissues. The ultrasound-modulated optical signal of the tumors has the potential for early detection of breast cancers and for assessing and monitoring treatment, because optical properties are related to tissue constituents and their molecular conformations. Recent studies have demonstrated that objects buried in dense turbid media (such as biological tissues) can be imaged using ultrasound-modulated optical tomography. Ultrasound focused into the tissue modulates the laser light passing through the focal spot. The ultrasound-modulated optical signal reflects the local mechanical and optical properties at the focal spot. Buried objects are located by scanning the device and detecting alterations of the ultrasound-modulated optical signal. So far, the applicants have achieved 1-mm imaging resolution in a 5-cm thick tissue phantom. This research has indicated the potential for clinical application in breast imaging and cancer detection. The differences in mechanical and optical properties between normal and abnormal tissues will be the basis of detecting tumors using this technique. To improve the imaging quality of single-frequency ultrasound-modulated optical images, the applicants have designed a refined imaging technique: frequency-swept ultrasound-modulated optical tomography, which minimizes the interference originating from off the ultrasonic focus. The specific aims of the proposed studies on frequency-swept ultrasound-modulate optical tomography include: 1.) Imaging objects in tissue phantoms; 2.) Imaging tumors in biological tissue in vitro; and 3.) Imaging breast tumors in vivo and measuring hemoglobin oxygenation saturation of tumors.